One of the problems faced by modern technology is that of delivering to the surface of the earth liquids located in subsurface pockets. It is customary in oil producing areas, for example, to drill a well from the earth's surface to a depth at which it penetrates into a natural reservoir of oil. That depth may be several hundred feet, much too great to permit operation of a simple suction pump, and a lift pump is installed near the bottom of the well, and connected by a string of sucker rods with surface apparatus, called a pumping jack, for causing the pump to reciprocate. Pumping jacks may be either mechanical, such as the familiar walking beam, or may be hydraulic. In either case, the pumping jack must be capable of the considerable stroke required, seven feet for example, and it it must be of sufficient power to move the string of sucker rods and the column of oil rising from the pump to the surface. Twelve thousand pounds may be a reasonable design load for such an apparatus.
Hydraulic pumping jacks of this sort are known, but suffer from certain imperfections. In the first place, a limited quantity of the hydraulic fluid--a liquid--is usually pumped from a reservoir into a hydraulic motor to raise the load, and then allowed to flow back into the reservoir so that the load descends by its own weight. Continuous repetition of this cycle subjects the hydraulic fluid to damage by heating. The arrangement also subjects the mechanical equipment to severe shock each time the direction of movement reverses from upward to downward, and it is further difficult to provide for varying the speed of operation of such equipment.